Since ZSM-5 type molecular sieves were first reported in 1972 by Mobil Corp., US (U.S. Pat. No. 3,702,886), they have been widely applied in various fields such as petrochemical industry, fine chemical industry, and environmental protection due to their unique three-dimensional channel structures and shape-selective property as well as advantages of high silica-to-alumina ratio, lipophilicity and hydrophobicity, high thermal stability and hydrothermal stability, and high catalytic activity.
Synthesis methods of molecular sieves may be divided into two categories on the basis of sources of raw materials: synthesis with chemical reagents, and synthesis with natural minerals. Processes for synthesizing molecular sieves with conventional inorganic chemical reagents as raw materials are mature technology with easily controllable process conditions and high product quality. However, most of these inorganic chemical reagents are prepared from natural minerals through complicated reactions and separation procedures, with a long process route as well as high material and energy consumption, and most of the processes result in substantial emission of pollutants. Hence, if molecular sieves can be synthesized directly from silicon- and aluminum-rich natural minerals as raw materials, not only is there a wide range of sources of raw materials, but also the synthesis route from raw materials to molecular sieve products can be greatly shortened, and energy consumption, material consumption, and pollutant emission can be significantly reduced, and production cost can be remarkably lowered, so as to discover new approaches for molecular sieve synthesis, which shows great promise in development. To date, public reports on molecular sieve synthesis with natural minerals as raw materials are primarily focused on molecular sieve synthesis with natural minerals such as kaolin and rectorite as part of the silicon source or aluminum source.
Kaolin is a 1:1 type dioctohedral layered aluminosilicate clay mineral, with a typical chemical composition Al2O3.2SiO2.2H2O, and therefore may be used as the silicon source and aluminum source for molecular sieve synthesis. Rectorite has basic structural units of silica tetrahedra and alumina octahedra, wherein, in the unit layer, tetrahedral sheets link to octahedral sheets in a 1:1 or 2:1 fashion, with a chemical formula of its unit cell of Al4[Si8O20](OH)4. As such, rectorite may also be used as the silicon source and aluminum source for molecular sieve synthesis.
U.S. Pat. No. 6,908,603 discloses a method for in situ synthesis of ZSM-5 molecular sieves on kaolin microspheres. In this method, the reaction mixture comprises calcinated kaolin microspheres, crystal seed solution for promoting Y zeolite and silicate, and this reaction mixture has a silica-to-alumina molar ratio of more than 20 and a pH higher than 14. The above mixture is reacted for a period of time at a certain temperature to provide ZSM-5 crystals grown in situ on kaolin microspheres. No organic template or ZSM-5 crystal seed is present in the reaction mixture.
U.S. Pat. No. 4,091,007 discloses a method of preparing ZSM-5 molecular sieves with kaolin. In this method, the reaction mixture comprises alkali metal ions, tetrapropylammonium, silicon source, aluminum source, and water, in which the aluminum source is supplied by kaolin. Such a reaction mixture is crystallized at 75 to 205° C. for 1 hour to 60 days, and ZSM-5 molecular sieves may thus be obtained.
EP 0068817 makes public a synthesis method of ZSM-5 molecular sieves in which calcinated kaolin is extracted with acid before it is used as an aluminum source, and crystallized at a certain temperature and under certain pressure for 1 to 2 days, with quaternary ammonium as a template, to afford ZSM-5 molecular sieve crystals, wherein the temperature for crystallization is no higher than 200° C.
CN 101332995A discloses a method for preparing ZSM-5 molecular sieves based on in situ crystallization of kaolin, which is characterized in that ZSM-5 molecular sieves grow in a in situ crystallization manner on the inner and outer surfaces of modified kaolin, the ZSM-5 molecular sieve in the resultant composite has a relative crystallinity of 30% to 80%, and the in situ crystallized ZSM-5 molecular sieve thus obtained has characteristics including high crystallinity, small crystal size, high activity and good stability.
CN 101462740A discloses a method for preparing ZSM-5 molecular sieves based on in situ crystallization of kaolin. In this method, a kaolin slurry comprising kaolin, a binder and water is spray dried to prepare kaolin microspheres, and the prepared kaolin microspheres are calcinated; further, a silicon-rich clay slurry comprising silicon-rich clay, a binder and water is spray dried to prepare silicon-rich clay microspheres, and then calcinated; after such separate calcination, the kaolin microspheres and the silicon-rich clay microspheres are mixed with a base-containing aqueous solution, and the resultant mixture is hydrothermally crystallized to provide a crystallized product. The ZSM-5 molecular sieve product prepared by the above method, by reason of its high content of zeolite and abundant mesopores, may be used as a catalyst in the field of petrochemical industry.
Honghong SHAN (Applied Clay Science, 2009, 42:439-445) synthesized a ZSM-5 molecular sieve with kaolin calcinated at 700° C. as a raw material. The obtained ZSM-5 molecular sieves have a slate-like morphology with a length of about 10 μm, and their silica-to-alumina molar ratio are 7.7 to 32.5.
CN 101722024A discloses a ZSM-5 molecular sieve/rectorite composite material and the preparation method thereof. The composite material is a crystal product formed via in situ crystallization by using natural rectorite minerals which provide the aluminum source and part of the silicon source for molecular sieve synthesis and is also used as the matrix for molecular sieve growth. In this composite material, the percentage content by mass of the ZSM-5 molecular sieve is no less than 10%, and the ZSM-5 molecular sieve has a silica-to-alumina ratio of 20 to 60. The composite material prepared by this method has excellent hydrothermal stability.
In all of the above patents or literatures in which kaolin or rectorite minerals are used as raw material for preparing ZSM-5 molecular sieve, it is necessary to externally add a part of chemical silicon source (e.g., silica sol, water glass, sodium silicate etc.) in order to adjust the silica-to-alumina ratio in the system. The reason therefor lies in that natural mineral raw materials are mostly activated at high temperature in the about methods, and as a result, alumina octahedra therein are completely destructed, while silica tetrahedra are still preserved in the original layered structure of the mineral, which can only provide aluminum source and a small quantity of silicon source, and hence a large amount of chemical silicon source needs to be added so as to meet the requirement for the silica-to-alumina ratio of the target molecular sieve. Further, when ZSM-5 molecular sieves are prepared with the methods provided in the above patents or literatures, there remains a large amount of unreacted kaolin or rectorite residues, which impacts the crystallinity and purity of the molecular sieve. In recent years, as green chemistry develops, research and development in new chemical process is focusing on use of non-toxic and harmless raw materials, improvement in raw material utilization, lowering of energy consumption in production process, and reduction in pollutant emission. In the field of molecular sieve synthesis, if silicon and aluminum sources in natural minerals can be fully employed and natural minerals that are rich in amorphous silica are used as external silicon sources to adjust the feed ratio of silica to alumina, it is desirable to establish an environmentally friendly novel technical route for molecular sieve synthesis.
Diatomite is a biological sedimentary rock, which is preserved in a form of diatom remains that is formed by diatom biologically absorbing soluble silica in water under certain physicochemical conditions including light, temperature and nutrients. Diatomite has a general theoretical formula of Mg8[Si12O30](OH)4(OH2)4.8H2O, which is a 2:1 type chained layer structure. Chemical components in diatomite are primarily amorphous SiO2, which may be used as the silicon source for molecular sieve synthesis after pretreatment such as purification and activation.
At present, synthesis of ZSM-5 type molecular sieves with natural minerals as raw materials has been reported, but only part of the silicon and aluminum in the mineral is utilized and a certain amount of external chemical silicon source or aluminum source is still needed to adjust silica-to-alumina ratio. It has not been reported that ZSM-5 type molecular sieves are synthesized with natural minerals as the total raw material without external addition of chemical silicon source or aluminum source. ZSM-5 type molecular sieves are molecular sieve materials widely used in the field of petrochemical industry, and development of techniques for synthesizing ZSM-5 type molecular sieves with natural minerals as the total raw material has great application prospect.